The precise role of the basal ganglia in motor control is still unclear. Profound deficits in both movement initiation and execution occur in Parkinson's disease patients and experimental animals, especially during ballistic movements and when behavioral requirements and outcome are unpredictable. Both patients and experimental animals also have deficits in proprioceptive control. Current hypotheses suggest that the basal ganglia play a role in movement initiation by "releasing" direct thalamocortical motor pathways thus "disinhibiting" cortex so that prepared movements can be initiated and new motor plans made. These hypotheses also suggest that indirect pathways suppress unwanted movements by suppressing other regions of cortex not directly associated with the motor behavior. Facilitation and suppression of the cortex during movement initiation and executive may alter the responsiveness of sensory neurons in cortical areas that provide input to the basal ganglia. Single-unit electrophysiological techniques will be used to record from striatal neurons from behaving monkeys while they perform hand movement tasks which result in changes in cortical neuronal responsiveness. The specific goals of proposed studies are 1) to understand the influence of unpredictable behavioral requirements upon striatal activity, 2) to understand the role that striatal neurons play in controlling the initiation and execution of proprioceptively guided movements and 3) to determine if a striatal neuron's activation history influences the time of onset and magnitude of stimulus and movement related activity. The results of the proposed experiments will be compared with those of ongoing studies in which sensorimotor cortical neurons are recorded under the same behavioral conditions. This comparison will determine if changes in striatal activity occur before those in cortex, thus suggesting the striatum may influence cortical responsiveness. The long-term goal of this work is to better understand the role the basal ganglia play in motor control during stimulus detection and classification, response programming and selection (initiation) and response production (execution). It is hoped that it will provide additional evidence to support the view that the basal ganglia influences motor control by regulating proprioceptive information processing in sensorimotor centers. Insights from these experiments will be crucial in understanding the cause and extent of motor deficits that accompany basal ganglia dysfunction.